11. Serial transmission is not possible without ________
TDM
FDM
PDM
None of the above
Answer.1. TDM
Explanation
Time-division multiplexing of multiple signals is possible only when the available data rate of the channel exceeds the data rate of the total number of users. While TDM may be applied to either digital or analog signals, in practice it is applied almost always to digital signals.
The data transmission may be serial or parallel. In serial transmission, bits are transmitted serially whereas in parallel transmission, all bits of a byte are transmitted at a time.
Parallel transmission is used for the devices which are close to each other whereas serial transmission is used for transmission over long distances. The communication modes allow for one-way, two-way ordered, or two-way simultaneous transmission.
In serial transmission, the data is sent via a single cable. When a clock pulse is applied to the shift register it transmits the information bit by bit in allocated time slots.
12. A TDM link has 24 signals and each channel is sampled 8000 times/sec. Each sample is represented by 8 binary bits and it contains an additional bit for synchronization. The total bit rate made from the TDM link is
1600 K bits/sec
1728 K bits/sec
1826 K bits/sec
2056 K bits/sec
Answer.2. 1728 K bits/sec
Explanation
Given sampling rate of channel each = 8000 Hz = fch
Since 24 such signals are Multiplexed, we get a total pulse per second (frequency) of the multiplied signal as:
fs = nfch
Putting on the respective values:
fs = 24 × 8000
fs = 192,000 = 192 KHz
Now each sample is represented by 8 bits and contains an additional bit for synchronization (n).
The total no. of bits per sample will be:
n = 8 + 1 = 9 bits
∴ The total bit rate of TDM link is given as:
Rb = nfs
Rb = 9 × 192 Kbps
Rb = 1728 Kilo-bits/sec
13. 24 voice channels are sampled uniformly at a rate of 8 kHz and then time division multiplexed. The sampling process uses flat-top samples with 1 μs duration. The multiplexing operation includes the provision of synchronization by adding an extra pulse of 1 μs duration. The spacing between successive pulses of the multiplexed signal is:
4 μs
6 μs
7.2 μs
8.4 μs
Answer.1. 4 μs
Explanation
Given that,
The total number of channels is the sum of the voice channels and synchronized samples
Total number of samples n = 25
Sampling frequency fs = 8 kHz
Sampling time Ts = 1 / fs
⇒ Ts = 1 / 8000
∴ Ts = 125 μs.
Time duration utilized by the samples is
T = Ts / n
⇒ T = 125 / 25
T = 5 μs
The pulse duration is 1μs
The spacing between successive pulses of the multiplexed signal is = 5 – 1 = 4 μs
14. The main advantage of time-division multiplexing (TDM) over frequency division multiplexing (FDM) is that it:
Requires less bandwidth
Requires less power
Requires simple circuitry
Provide better signal-to-noise ratio
Answer.4. Provide better signal-to-noise ratio
Explanation
TDM (Time Division Multiplexing):
It is a multiplexing technique that allows the transmission of multiple signals over a common channel but in different time slots.
Each signal will get transmitted very quickly over the channel but at a time only one signal will be transmitted.
FDM (Frequency Division Multiplexing):
It is also a multiplexing technique like TDM, in which multiple transmitted signals use a common channel but the total available bandwidth is utilized among the various signals.
This implies that over a complete channel a particular frequency slot is allotted to only one signal.
In FDM, a different frequency band is used to modulate different data signals. This means that different carrier frequency modulates the various signals that are to be transmitted over the channel.
Key differences between FDM and TDM:
FDM
TDM
It is used in Analog signal transmission
It is used in Digital signal transmission
In FDM the signals are allotted different frequency slots, this leads to the possibility of crosstalk while signal transmission.
In TDM multiple signals are transmitted in different time domains, thus the chances of crosstalk between the signals are negligible
The Signal-Noise ratio is higher
Provide better Signal-Noise ratio
Synchronization is not needed
Synchronization is necessary
Circuit orientation is quite complex
Circuit orientation is comparatively simple.
Costlier
Comparatively less
FDM offers low efficiency because the technique is more prone to cause interference while transmission.
TDM is highly efficient because the chances of interference are less in its case.
No propagation delay
Propagation delays are present
15. Random-access Protocol is
Distributed
Fault-tolerant
Distributed & Fault-tolerant
None of the mentioned
Answer.3. Distributed & Fault-tolerant
Explanation
Random Access protocols deal with the access to the transmission medium in communication networks, as part of the medium access control layer. Random access is simple, distributed, and very fault-tolerant.
16. What device is used to demodulate a time division multiplexed analog wave?
High pass filter
Low pass filter
Bandstop filter
Attenuator
Answer.2. Low pass filter
Explanation
In TDM, each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a short period of time. A rotating switch called a commutator connects the output of each channel modulator to the input of the communication channel in turn.
The commutator is realized with electronic switches since it has to rotate at high speeds. The commutator remains at each contact for a fixed interval of time, which is the time slot allotted for each channel.
At the receiver, another switch is used, rotating in synchronization with the commutator in the sending end. This switch connects the pulses received to the appropriate demodulator.
A low-pass filter in conjunction with a diode acts as a simple demodulator for an amplitude-modulated wave.
The original analog signal is recovered by passing it through a low-pass filter. In TDM using PAM, a circuit called a multiplexer (MUX or MPX) samples multiple analog signal sources; the resulting pulses are interleaved and then transmitted over a single channel.
17. Four voice signals, each limited to 4 kHz and sampled at Nyquist rate, are converted into binary PCM signals using 256 quantization levels. The bit transmission rate for the time-division multiplexing signal will be
8 kbps
64 kbps
256 kbps
5126 kbps
Answer.3. 256 kbps
Explanation
Transmission rate (R = nmfs
n → number of bits in the signal
m → number of signals
fs → sampling frequency
Calculation:
Given:
No. of levels of PCM = 256
∴ no. of bits = log2 256
= 8 bits
m = 4
fs = (2fm) = 8 kHz
(R = (8 × 4 × 8)
(R = 256 kbps
18. What multiplexing method do the CODECs use to transmit two PCM signals in opposite directions over the same transmission line?
TDM
FDM
Amplitude-separation Multiplexing
Code Division Multiplexing
Answer.1. TDM
Explanation
CODEC is a device that can simultaneously perform the analog-to-digital conversion (encoding) and digital-to-analog conversion (decoding).
Time Division Multiplexing (TDM)
A process that transmits two or more message signals over the same line by using a different time interval for each signal.
Time-division multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel.
TDM is used to transmit two PCM signals in opposite directions over the same transmission line.
19. The different access methods which permit many satellite users to operate in parallel through a single transponder without interfering with each other are
1. Frequency Division Multiple Access (FDM
2. Time Division Multiple Access (TDM
3. Code Division Multiple Access (CDM
Which of the above are correct?
1 and 2 only
1 and 3 only
2 and 3 only
1, 2 and 3
Answer.2. 1 and 3 only
Explanation
In TDM, each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a short period of time.
In TDMA, different users access the transponder in different time slots, not in parallel. In serial transmission is used for transmission over long distances. The communication modes allow for one-way, two-way ordered, or two-way simultaneous transmission.
In serial transmission, the data is sent via a single cable. When a clock pulse is applied to the shift register it transmits the information bit by bit in allocated time slots.
20. Example of a reservation system is
Synchronous TDM
Non-synchronous TDM
Synchronous & Non-synchronous TDM
None of the mentioned
Answer.1. Synchronous TDM
Explanation
A Synchronous TDM is a technique in which a time slot is preassigned to every device. In Synchronous TDM, each device is given some time slot irrespective of the fact that the device contains the data or not. If the device does not have any data, then the slot will remain empty.
In the reservation scheme, a node is required to make a reservation prior to transmitting data. Time is split into intervals. In each interval, a reservation frame is initially sent.
Assume that there is n number of nodes. In this case, the reservation frame has precisely n number of mini time slots. Each mini time slot corresponds to a node.
If a node has sufficient data to send, it indicates its intention in its corresponding mini time slot. The nodes that made reservations, then transmit their data frames subsequent to the transmission of the reservation frame.
Synchronous TDM is an example of the reservation system